1. Field of the Invention
The present invention relates to a stabilized power source circuit incorporated in an IC and an IC itself. More specifically, the present invention relates to; an IC incorporating a power source circuit which facilitates design of a stabilized power source circuit having a required rated output current, and further facilitates the layout thereof in relation to other circuits to be integrated and is able to supply current with plenty of room with respect to a maximum supply current.
2. Description of Related Art
FIG. 9 shows an example of conventional stabilized power source circuits incorporated in an IC. Further, FIG. 10 shows a stabilized power source circuit in which an over current detecting transistor Qp' having 1/n emitter area is provided of which a base is connected in common with the base of an output transistor (power transistor) Qp.
In the both drawings, the stabilized power source circuits include a reference voltage source 1 which generates reference voltage Vr, an output voltage detection circuit 2 which generates a detection voltage Vf by dividing the output voltage Vo of the stabilized power source circuit with resistors R1 and R2, and an error amplifying circuit 3 which amplifies an error component between the reference voltage Vr and the detection voltage Vf and outputs the same. These circuits further include a transistor Qp which receives at the base thereof the output from the error amplifying circuit 3 via control of a current control circuit 5 and an over current detection circuit 4 which detects an over current of the output current from the transistor Qp and sends out a detection signal to the current control circuit 5.
In the stabilized power source circuits, the transistor Qp functions as a regulation transistor which lowers the source voltage Vcc down to the output voltage Vo depending on the output from the error amplifying circuit 3. In FIG. 10, when the transistor Qp' operates together with the regulation transistor Qp and detects a 1/n over current at 1/n collector current of the transistor Qp at the transistor Qp', an over current of the output transistor Qp is detected.
With these circuits an error signal A corresponding to a deviation amount of the output voltage Vo from a target value is generated so as to perform a control to achieve Vf=Vr, in that the output voltage Vo is controlled so as to reach the target value (Vr.times.(R1+R2)/R2) of a constant value. Even when the power source voltage Vcc fluctuates or when the condition of the load side receiving the output voltage Vo varies, the output voltage Vo is more or less stabilized at the target value.
Further, the error signal is in principle generated depending on the difference of the output voltage Vo from the target value, however in practice, a predetermined rate, for example R2/(R1+R2) is generally multiplied to the output voltage Vo to determine a lower detection voltage Vf, likely the same rate R2/(R1+R2) is multiplied to the target voltage value to determine a lower reference voltage Vr, then a control is performed so as to coincide these two lowered voltages while generating an error signal thereof.
The over current detection circuit 4 detects the moment when a current more than a rated current of the transistor Qp flows through the collector of the transistor Qp. The current control circuit 5 is for controlling the operating current of the error amplifying circuit 3 in response to the detection signal, and is normally fed from the current source of the error amplifying circuit 3. These circuits detect a moment when the output current via the transistor Qp or the transistor Qp' has exceeding or immediately before exceeds the rated current of the transistor Qp by the over current detection circuit 4 and limit the output current while suppressing the operating current of the error amplifying circuit 3 by the current control circuit 5. Namely, these circuits serve as a protective circuit for the transistor Qp.
When an over current is detected, amplification rate of the error amplifier 3 reduces, an increase of the error signal A is suppressed and an increase of the output current is thus suppressed prior to stabilization of the output voltage Vo. As a result, the output current is limited to be equal to or less than the rated output current value of the output transistor Qp. Thereby, breakdown of the output transistor Qp as well as an IC incorporating the same is prevented. Further, the target value determined with regard to the output voltage Vo is normally set within a range not exceeding the rated current.
In these conventional stabilized power source circuits incorporated into an IC, a power transistor is used for controlling the output voltage. An area of the transistor region of a power transistor is designed to be large for permitting a large current flow so that the power transistor is required to have measures for preventing a local current concentration and for making the current density uniform. Further, the power transistor is required to be designed to make the wiring resistance uniform and to suppress process variation. On one hand, in such power transistor, the occupying area increases depending on an increase of the rated current which makes the layout thereof difficult and requires long design hours.
Further, since such operating characteristics as a possible current flow capacity and a safe operating region of a power transistor are not simply proportional to the area of the transistor region, there remained a problem that an accurate performance of a stabilized power source circuit can not be determined before producing experimental pieces and checking the actual operating results thereof. With regard to such power transistors the design efficiency is low and the development risk thereof is high. Further, for the rated current determined for a power transistor there appears to be variation such that the rated output current has to be designed with some room. However, in response to the provision of such room, the area occupying rate thereof in the integrated circuit increases and the layout thereof becomes difficult in connection with other circuits to be integrated. Further, in response to the scale increase of other circuits to be integrated, the rated output current of the power transistor increases and the occupying area of the stabilized power source circuit increases which makes it more and more difficult to form the same in a single chip IC.